Mesoscale eddies are important to many aspects of the dynamics of the Arctic Ocean. Among others, they maintain the halocline and interact with the Atlantic Water circumpolar boundary current through lateral eddy fluxes and shelf-basin exchanges. Mesoscale eddies are also important for transporting biological material and for modifying sea ice distribution. Here, we review what is known about eddies and their impacts in the Arctic Ocean in the context of rapid climate change. Eddy kinetic energy (EKE) is a proxy for mesoscale variability in the ocean due to eddies. We present the first quantification of EKE from moored observations across the entire Arctic Ocean and compare those results to output from an eddy resolving numerical model. We show that EKE is largest in the northern Nordic Seas/Fram Strait and it is also elevated along the shelf break of the Arctic Circumpolar Boundary Current, especially in the Beaufort Sea. In the central basins, EKE is 100–1,000 times lower. Generally, EKE is stronger when sea ice concentration is low versus times of dense ice cover. As sea ice declines, we anticipate that areas in the Arctic Ocean where conditions typical of the North Atlantic and North Pacific prevail will increase. We conclude that the future Arctic Ocean will feature more energetic mesoscale variability.
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The Contrasting Dynamics of the Buoyancy-Forced Lofoten and Greenland Basins
The Nordic seas are commonly described as a single basin to investigate their dynamics and sensitivity to environmental changes when using a theoretical framework. Here, we introduce a conceptual model for a two-basin marginal sea that better represents the Nordic seas geometry. In our conceptual model, the marginal sea is characterized by both a cyclonic boundary current and a front current as a result of different hydrographic properties east and west of the midocean ridge. The theory is compared to idealized model simulations and shows good agreement over a wide range of parameter settings, indicating that the physics in the two-basin marginal sea is well captured by the conceptual model. The balances between the atmospheric buoyancy forcing and the lateral eddy heat fluxes from the boundary current and the front current differ between the Lofoten and the Greenland Basins, since the Lofoten Basin is more strongly eddy dominated. Results show that this asymmetric sensitivity leads to opposing responses depending on the strength of the atmospheric buoyancy forcing. Additionally, the front current plays an essential role for the heat and volume budget of the two basins, by providing an additional pathway for heat toward the interior of both basins via lateral eddy heat fluxes. The variability of the temperature difference between east and west influences the strength of the different flow branches through the marginal sea and provides a dynamical explanation for the observed correlation between the front current and the slope current of the Norwegian Atlantic Current in the Nordic seas.
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- NSF-PAR ID:
- 10163007
- Date Published:
- Journal Name:
- Journal of Physical Oceanography
- Volume:
- 50
- Issue:
- 5
- ISSN:
- 0022-3670
- Page Range / eLocation ID:
- 1227 to 1244
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The circulation within marginal seas subject to periodic winds, and their exchange with the open ocean, are explored using idealized numerical models and theory. This is motivated by the strong seasonal cycle in winds over the Nordic Seas and the exchange with the subpolar North Atlantic Ocean through the Denmark Strait and Faroe Bank Channel. Two distinct regimes are identified: an interior with closed
f /h contours and a shallow shelf region that connects to the open ocean. The interior develops a strong oscillating along-topography circulation with weaker ageostrophic radial flows. The relative importance of the bottom Ekman layer and interior ageostrophic flows depends only onωh /Cd , whereω is the forcing frequency,h is the bottom depth, andCd is a linear bottom drag coefficient. The dynamics on the shelf are controlled by the frictional decay of coastal waves over an along-shelf scaleLy =f 0LsHs /Cd , wheref 0is the Coriolis parameter, andLs andHs are the shelf width and depth. ForLy much less than the perimeter of the basin, the surface Ekman transport is provided primarily by overturning within the marginal sea and there is little exchange with the open ocean. ForLy on the order of the basin perimeter or larger, most of the Ekman transport is provided from outside the marginal sea with an opposite exchange through the deep part of the strait. This demonstrates a direct connection between the dynamics of coastal waves on the shelf and the exchange of deep waters through the strait, some of which is derived from below sill depth.Significance Statement The purpose of this study is to understand how winds over marginal seas, which are semienclosed bodies of water around the perimeter of ocean basins, can force an exchange of water, heat, salt, and other tracers through narrow straits between the marginal sea and the open ocean. Understanding this exchange is important because marginal seas are often regions of net heat, freshwater, and carbon exchange with the atmosphere. The present results identify a direct connection between processes along the coast of the marginal sea and the flow of waters through deep straits into the open ocean.
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null (Ed.)The marine-based West Antarctic Ice Sheet (WAIS) is currently retreating due to shifting wind-driven oceanic currents that transport warm waters toward the ice margin, resulting in ice shelf thinning and accelerated mass loss of the WAIS. Previous results from geologic drilling on Antarctica’s continental margins show significant variability in marine-based ice sheet extent during the late Neogene and Quaternary. Numerical models indicate a fundamental role for oceanic heat in controlling this variability over at least the past 20 My. Although evidence for past ice sheet variability has been collected in marginal settings, sedimentologic sequences from the outer continental shelf are required to evaluate the extent of past ice sheet variability and the associated oceanic forcings and feedbacks. International Ocean Discovery Program Expedition 374 drilled a latitudinal and depth transect of five drill sites from the outer continental shelf to rise in the eastern Ross Sea to resolve the relationship between climatic and oceanic change and WAIS evolution through the Neogene and Quaternary. This location was selected because numerical ice sheet models indicate that this sector of Antarctica is highly sensitive to changes in ocean heat flux. The expedition was designed for optimal data-model integration and will enable an improved understanding of the sensitivity of Antarctic Ice Sheet (AIS) mass balance during warmer-than-present climates (e.g., the Pleistocene “super interglacials,” the mid-Pliocene, and the late early to middle Miocene). The principal goals of Expedition 374 were to • Evaluate the contribution of West Antarctica to far-field ice volume and sea level estimates; • Reconstruct ice-proximal atmospheric and oceanic temperatures to identify past polar amplification and assess its forcings and feedbacks; • Assess the role of oceanic forcing (e.g., sea level and temperature) on AIS stability/instability; • Identify the sensitivity of the AIS to Earth’s orbital configuration under a variety of climate boundary conditions; and • Reconstruct eastern Ross Sea paleobathymetry to examine relationships between seafloor geometry, ice sheet stability/instability, and global climate. 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We moved to a nearby alternate site on the continental slope (U1525) to core a single hole with a record complementary to the upper part of the section recovered at Site U1524. We returned to Site U1524 3 days later, after the sea ice cleared. We then cored Hole U1524C with the rotary core barrel with the intention of reaching the target depth of 1000 m DSF. However, we were forced to terminate Hole U1524C at 441.9 m DSF due to a mechanical failure with the vessel that resulted in termination of all drilling operations and a return to Lyttelton 16 days earlier than scheduled. The loss of 39% of our operational days significantly impacted our ability to achieve all Expedition 374 objectives as originally planned. 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Abstract The Atlantic Meridional Overturning Circulation (AMOC), a key mechanism in the climate system, delivers warm and salty waters from the subtropical gyre to the subpolar gyre and Nordic Seas, where they are transformed into denser waters flowing southward in the lower AMOC limb. The prevailing hypothesis is that dense waters formed in the Labrador and Nordic Seas are the sources for the AMOC lower limb. However, recent observations reveal that convection in the Labrador Sea contributes minimally to the total overturning of the subpolar gyre. In this study, we show that the AMOC is instead primarily composed of waters formed in the Nordic Seas and Irminger and Iceland basins. A first direct estimate of heat and freshwater fluxes over these basins demonstrates that buoyancy forcing during the winter months can almost wholly account for the dense waters of the subpolar North Atlantic that are exported as part of the AMOC.
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Abstract. The overturning streamfunction as measured at the OSNAP (Overturning in the Subpolar North Atlantic Program) mooring array represents the transformation of warm, salty Atlantic Water into cold, fresh North Atlantic Deep Water (NADW). The magnitude of the overturning at the OSNAP array can therefore be linked to the transformation by air–sea buoyancy fluxes and mixing in the region north of the OSNAP array. Here, we estimate these water mass transformations using observational-based, reanalysis-based and model-based datasets. Our results highlight that air–sea fluxes alone cannot account for the time-mean magnitude of the overturning at OSNAP, and therefore a residual mixing-driven transformation is required to explain the difference. A cooling by air–sea heat fluxes and a mixing-driven freshening in the Nordic Seas, Iceland Basin and Irminger Sea precondition the warm, salty Atlantic Water, forming subpolar mode water classes in the subpolar North Atlantic. Mixing in the interior of the Nordic Seas, over the Greenland–Scotland Ridge and along the boundaries of the Irminger Sea and Iceland Basin drive a water mass transformation that leads to the convergence of volume in the water mass classes associated with NADW. Air–sea buoyancy fluxes and mixing therefore play key and complementary roles in setting the magnitude of the overturning within the subpolar North Atlantic and Nordic Seas. This study highlights that, for ocean and climate models to realistically simulate the overturning circulation in the North Atlantic, the small-scale processes that lead to the mixing-driven formation of NADW must be adequately represented within the model's parameterisation scheme.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1175/JPO-D-19-0280.1
